SUDOKU #6  
 

I'm going to repeat the info at the top of Sudoku #1, just in case you haven't seen that one yet.
 
I've been doing Sudoku for a while. There are a lot of people like me who have figured out some tricks or techniques that help them complete the puzzles. This page is not really for them. I met a woman recently who was just starting to try to do Sudoku. This page is for people like her. I'm not saying that I know every single trick, but I know enough to complete most puzzles.
 
The basic rules of Sudoku are that there is only one of each number in each row, column, and 9 square grid. There isn't any mathematics or arithmetic. If you know how to count from 1 to 9, then you can do these puzzles. All of the techniques are based on this rule.
 
Here is a sample Sudoku puzzle.
 
              3     2  
    9       1 2        
    3 1     7 9 4 5    
    6 5         2 7    
    7               4  
      9 4         3 6  
      3 7 9 4     2 5  
          7 2       9  
    2     3            
 
This is a Medium difficulty puzzle, and it will allow me to show some examples of a few of the tricks that I've uncovered. I've added the letters below to the rows and columns in order to talk about each of the 81 squares. As a convention, I will talk about squares and boxes. There are 81 squares, but there are 9 boxes of 9 squares.
     
  A           3     2  
  B 9       1 2        
  C 3 1     7 9 4 5    
  D 6 5         2 7    
  E 7               4  
  F   9 4         3 6  
  G   3 7 9 4     2 5  
  H       7 2       9  
  I 2     3            
  J K L M N O P Q R  
                         
Notice that there aren't any 8s. There is a 2 in CK (because of AR and BO), EK (because of CL and JI), and FM (because of DP, EK, BO, HN). That finished the 2s.
         
  A           3     2    
  B 9       1 2          
  C 3 1 2   7 9 4 5      
  D 6 5         2 7      
  E 7 2             4    
  F   9 4 2       3 6    
  G   3 7 9 4     2 5    
  H       7 2       9    
  I 2     3              
    J K L M N O P Q R    
                             
There is a 9 in IL (because of BJ, FK, GM, and HR). There has to be a 9 in either EP or EQ (because of HR and FK), so there has to be a 9 in DN (because of EP/EQ, FK, CO, and GM).
             
  A           3     2    
  B 9       1 2          
  C 3 1 2   7 9 4 5      
  D 6 5     9   2 7      
  E 7 2             4    
  F   9 4 2       3 6    
  G   3 7 9 4     2 5    
  H       7 2       9    
  I 2   9 3              
    J K L M N O P Q R    
                         
One of the numbers missing from column R is a 3, but there can't be a 3 in IR (because IM), DR (because of FQ), and CR (because of CJ), so the 3 has to be in BR. There is a 3 is in HP (because of GK, IM, BR, and FQ), a 3 in EN (because of IM, AO, and FQ), and the final 3 is in DL (because of FQ and EN).
         
  A           3     2    
  B 9       1 2     3    
  C 3 1 2   7 9 4 5      
  D 6 5 3   9   2 7      
  E 7 2     3       4    
  F   9 4 2       3 6    
  G   3 7 9 4     2 5    
  H       7 2   3   9    
  I 2   9 3              
    J K L M N O P Q R    
                         
There is a 7 in IR (because of CN and DQ), a 1 in DR (because of CK), and an 8 in CR. And there is a 6 in CM.
         
  A           3     2    
  B 9       1 2     3    
  C 3 1 2 6 7 9 4 5 8    
  D 6 5 3   9   2 7 1    
  E 7 2     3       4    
  F   9 4 2       3 6    
  G   3 7 9 4     2 5    
  H       7 2   3   9    
  I 2   9 3         7    
    J K L M N O P Q R    
                         
There has to be a 1 in box GM-IO in column O (because of BN), so there has to be a 1 in EM (because of DR). That means that there is a 1 in FJ and an 8 in EL, and there is another 1 in HL (because of FJ and CK). And there is an 8 in FP.
         
  A           3     2    
  B 9       1 2     3    
  C 3 1 2 6 7 9 4 5 8    
  D 6 5 3   9   2 7 1    
  E 7 2 8 1 3       4    
  F 1 9 4 2     8 3 6    
  G   3 7 9 4     2 5    
  H     1 7 2   3   9    
  I 2   9 3         7    
    J K L M N O P Q R    
                         
There is a 4 in either AM or BM (because of BN), so there is a 4 in DO and an 8 in DM. That means there is an 8 in AN and another one in BK (because of EL and AN).
         
  A         8 3     2    
  B 9 8     1 2     3    
  C 3 1 2 6 7 9 4 5 8    
  D 6 5 3 8 9 4 2 7 1    
  E 7 2 8 1 3       4    
  F 1 9 4 2     8 3 6    
  G   3 7 9 4     2 5    
  H     1 7 2   3   9    
  I 2   9 3         7    
    J K L M N O P Q R    
                         
The numbers missing from column L are a 5 and 6, so the numbers in AJ and AK have to be a 4 and a 7. There is a 7 in EJ, so the 7 is in AK and the 4 is in AJ. That means there is a 4 in BM and a 5 in AM, which means that there is a 5 in BL and a 6 in AL.
         
  A 4 7 6 5 8 3     2    
  B 9 8 5 4 1 2     3    
  C 3 1 2 6 7 9 4 5 8    
  D 6 5 3 8 9 4 2 7 1    
  E 7 2 8 1 3       4    
  F 1 9 4 2     8 3 6    
  G   3 7 9 4     2 5    
  H     1 7 2   3   9    
  I 2   9 3         7    
    J K L M N O P Q R    
                         
There is a 7 in BP (because of AK and DQ), a 6 in BQ.
         
  A 4 7 6 5 8 3     2    
  B 9 8 5 4 1 2 7 6 3    
  C 3 1 2 6 7 9 4 5 8    
  D 6 5 3 8 9 4 2 7 1    
  E 7 2 8 1 3       4    
  F 1 9 4 2     8 3 6    
  G   3 7 9 4     2 5    
  H     1 7 2   3   9    
  I 2   9 3         7    
    J K L M N O P Q R    
                         
There is a 5 in EP (because of CQ), a 9 in EP, and a 6 in EO. So there is a 9 in AP and a 1 in AQ.
         
  A 4 7 6 5 8 3 9 1 2    
  B 9 8 5 4 1 2 7 6 3    
  C 3 1 2 6 7 9 4 5 8    
  D 6 5 3 8 9 4 2 7 1    
  E 7 2 8 1 3 6 5 9 4    
  F 1 9 4 2     8 3 6    
  G   3 7 9 4     2 5    
  H     1 7 2   3   9    
  I 2   9 3         7    
    J K L M N O P Q R    
                         
There is a 6 in IN (because of EO), a 6 in GP (because of IN and BQ), and a 6 in HK (because of DJ and IN). There is a 5 in FN, a 7 in FO, a 1 in IP, and a 4 in IK.
         
  A 4 7 6 5 8 3 9 1 2    
  B 9 8 5 4 1 2 7 6 3    
  C 3 1 2 6 7 9 4 5 8    
  D 6 5 3 8 9 4 2 7 1    
  E 7 2 8 1 3 6 5 9 4    
  F 1 9 4 2 5 7 8 3 6    
  G   3 7 9 4   6 2 5    
  H   6 1 7 2   3   9    
  I 2 4 9 3 6   1   7    
    J K L M N O P Q R    
                         
There is a 1 in GO (because of FJ), an 8 in GJ, and a 5 in HJ.
         
  A 4 7 6 5 8 3 9 1 2    
  B 9 8 5 4 1 2 7 6 3    
  C 3 1 2 6 7 9 4 5 8    
  D 6 5 3 8 9 4 2 7 1    
  E 7 2 8 1 3 6 5 9 4    
  F 1 9 4 2 5 7 8 3 6    
  G 8 3 7 9 4 1 6 2 5    
  H 5 6 1 7 2   3   9    
  I 2 4 9 3 6   1   7    
    J K L M N O P Q R    
                         
There is a 5 in IO, an 8 in IQ, an 8 in HO and a 4 in HQ.
         
  A 4 7 6 5 8 3 9 1 2    
  B 9 8 5 4 1 2 7 6 3    
  C 3 1 2 6 7 9 4 5 8    
  D 6 5 3 8 9 4 2 7 1    
  E 7 2 8 1 3 6 5 9 4    
  F 1 9 4 2 5 7 8 3 6    
  G 8 3 7 9 4 1 6 2 5    
  H 5 6 1 7 2 8 3 4 9    
  I 2 4 9 3 6 5 1 8 7    
    J K L M N O P Q R    
 
 
HOME